I. Field of the Invention
This invention relates generally to the construction of flat panel membrane-type switch arrays of the type now finding wide usage on a variety of household and industrial appliances and more specifically to the design of an improved composite top membrane for use with such switch arrays whose decorative cover layer is substantially free from unsightly ripples.
II. Discussion of the Prior Art
As mentioned above, flat panel membrane-type switch arrays are now finding wide application in a variety of electrical and electronic devices. Typically, a membrane-type switch comprises a base plate having a pattern of printed conductors thereon and overlaying this base plate is a spacer layer having apertures therethrough which are aligned with the pattern on the base plate. Disposed on the opposite side of the spacer layer is a resilient plastic material having a pattern of metallization on the undersurface thereof, also in substantial registration with the apertures formed in the spacer layer. Graphic information such as alphanumeric characters are frequently provided on the opposite surface of the metallized plastic layer and completing the assembly may be a top protective layer of clear plastic which permits the graphic information to be viewed by the user. To operate the switch, the user applies finger pressure to the indicia, thus forcing the metallized plastic layer through the spacer so as to contact the printed circuit pattern on the base plate. Once the finger pressure is removed, the "memory" property of the metallized plastic layer allows the metallization pattern to be withdrawn from contact with the printed circuit pattern on the base plate.
It is also common practice to adhesively bond the various layers one to the other. That is, the outer clear plastic layer may be bonded to the side of the metallized layer having the alphanumeric information printed on it and the spacer layer is then bonded to the side of the plastic layer bearing the metallization. Again, adhesive may be used to bond the undersurface of the spacer layer to the base plate having the printed circuit pattern on it.
One significant problem that has detracted from wider use of membrane-type switch arrays is the presence of unsightly ripples in the decorative cover layer. It is found that when the prior art approach is employed in constructing such panels, the outer plastic layers tend to follow the convolutions of the metallization pattern and depressions surrounding the spacer apertures such that light is reflected in a non-uniform manner and instead of appearing flat, the diaphragm switch array appears rippled and non-planar.
A typical prior art top assembly for a membrane-type switch is shown in the Riniker U.S. Pat. No. 4,035,593. In this arrangement the spacer layer is identified by numeral 20, the flexible membrane having the memory property is identified by numeral 14 and has a metallization pattern 16 on the undersurface thereof in substantial registration with the apertures formed in the spacer layer and has printed indicia (not shown) on the opposite surface thereof. Completing the assembly is a clear plastic cover layer 12 which protects the graphic material from wear. Adhesive layers 22 and 24 are provided for bonding the various layers or sheets together.
It is found that when switch panels of any appreciable size are constructed in this fashion, slight irregularities in the way of ripples, undulations and irregular depressions occur. Where the outer transparent protective layer has a glossy finish, these irregularities are highly noticable in that the light pattern reflected from the glossy surface highlights the fact that the surface is not perfectly smooth. It is, of course, desirable that the outward appearance of the glossy panel surface appear as if it were a sheet of smooth glass with no noticable irregularities.